Electromagnetic wave shielding sheet and printed wiring board

ABSTRACT

An electromagnetic wave shielding sheet according to the disclosure is configured by a protection layer, a metal layer, and a conductive adhesive layer. The metal layer has a plurality of openings, and an aperture ratio of the opening is 0.1%-20%. In addition, a tensile breaking strength of the electromagnetic wave shielding sheet is 10 N/20 mm-80 N/20 mm.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of patent application Ser. No.16/673,927, filed on Nov. 4, 2019, which claims the priority benefit ofJapanese Patent Application No. No. 2019-101051, filed on May 30, 2019and is now allowed. The entirety of each of the above-mentioned patentapplications is hereby incorporated by reference herein and made a partof this specification.

BACKGROUND Technical Field

The disclosure relates to an electromagnetic wave shielding sheetsuitable to be used by being bonded to one part of a component whichemits electromagnetic waves and relates to a printed wiring board withelectromagnetic wave shielding sheet.

In various electronic machines such as a portable terminal, a personalcomputer (PC), a server and the like, substrates such as printed wiringboards or the like are incorporated. An electromagnetic wave shieldingstructure is arranged on the substrate to prevent malfunction due to amagnetic field or a radio wave from outside and to reduce unnecessaryradiation from an electric signal.

In International Publication No. 2013/077108, in order to provide ashielding film, a shielding printed wiring board and a manufacturemethod of shielding film, the shielding film excellently blockingelectric field waves, magnetic field waves and electromagnetic waveswhich travel from one surface side to the other surface side of theshielding film and having a good transmission characteristic even whenapplied to a high frequency signal system, a configuration below isdisclosed. That is, a shielding film characterized in including a metallayer with a layer thickness of 0.5 μm-12 μm and an anisotropicconductive layer in a laminated state is disclosed. Moreover, it isrecited that the electric field waves, the magnetic field waves, and theelectromagnetic waves which travel from one surface side to the othersurface side of the shielding film can be excellently blocked by aconfiguration in which a ground circuit of the printed wiring board andthe anisotropic conductive layer are grounded.

In International Publication No. 2014/192494, in order to solve aproblem that an interlayer adhesion of a shielding printed wiring boardis damaged by a volatile component generated in a heating press orsolder reflow process, a shielding film for printed wiring boardincluding a metal foil having pinholes with a diameter of 0.1 μm-100 μmand a number of 10/cm²-1000/cm² in a metal film layer of anelectromagnetic wave shielding sheet and a conductive adhesive layer isrecited.

Along with a trend of high-speed transmission of a transmission signal,the electromagnetic wave shielding sheet is also required to have a highshielding property for high frequency and a transmission characteristicfor high frequency. Therefore, it is suitable to use the metal layer inthe conductive layer of the electromagnetic wave shielding sheet asrecited in International Publication No. 2013/077108.

However, when the shielding printed wiring board in which theelectromagnetic wave shielding sheet using the metal layer is attachedto the printed wiring board carries out a heating treatment such assolder reflow or the like, there is a problem that floating is generatedbetween layers due to the volatile component generated from the insideof the printed wiring board, and poor appearance and poor connection areresulted due to foaming or the like (hereinafter, sometimes referred toas solder reflow resistance).

In order to solve the problem, in International Publication No.2014/192494, it is proposed that a metal layer having pinholes isapplied to the electromagnetic wave shielding sheet; however, by thediameter and the number of the above pinholes, the solder reflowresistance cannot exhibit performance that can withstand practical use.

In addition, there is a problem that if the electromagnetic waveshielding sheet having the pinholes is thermally pressed, crackstriggered by the pinholes are generated in the metal layer and theelectromagnetic wave shielding property deteriorates (hereinafter,referred to as crack resistance).

In addition, along with miniaturization of the electronic machines, acircuit area of the printed wiring board is also reduced, and an openingarea of a via for ground-connection is miniaturized. In theelectromagnetic wave shielding sheets of International Publication No.2013/077108 and International Publication No. 2014/192494, reliabilityof the ground-connection with respect to the above small opening via ispoor and the electromagnetic wave shielding property deteriorates.

SUMMARY

The disclosure provides an electromagnetic wave shielding sheet and aprinted wiring board using the electromagnetic wave shielding sheet, theelectromagnetic wave shielding sheet having excellent solder reflowresistance and crack resistance, capable of high reliableground-connection even with respect to the small opening via, and havinga high electromagnetic wave shielding property even when used in a highfrequency transmission circuit.

As a result of intensive studies, the inventors found that the problemsof the disclosure can be solved in the following aspects and completedthe disclosure.

The electromagnetic wave shielding sheet of the disclosure is configuredby a protection layer, a metal layer, and a conductive adhesive layer,wherein the metal layer has a plurality of openings, an aperture ratioof the openings is 0.1%-20%, and a tensile breaking strength is 10 N/20mm-80 N/20 mm.

The above and other objects, features and advantages of the presentdisclosure will become more fully understood from the detaileddescription given hereinbelow and the accompanying drawings which aregiven by way of illustration only, and thus are not to be considered aslimiting the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section view of a cutoff portion showing oneexample of a printed wiring board according to the embodiment.

FIG. 2 is a schematic plan view of a principal surface side of a printedwiring board according to an example and a comparison example.

FIG. 3 is a schematic plan view of a back surface side of the printedwiring board according to the example and the comparison example.

FIG. 4 is a schematic plan view of a back surface side of the printedwiring board according to the example and the comparison example.

FIG. 5 is a schematic plan view of a principal surface side of a printedwiring board having a microstrip line circuit for crosstalk measurement.

FIG. 6 is a schematic plan view of a back surface side of the printedwiring board having a microstrip line circuit for crosstalk measurement.

FIG. 7 is a cross-section view of a cutoff portion along XI-XI of FIG.2.

FIG. 8 is a cross-section view of a cutoff portion along XII-XII of FIG.2.

FIGS. 9(1) to 9(6) are schematic views of a connection reliabilityevaluation to a small opening via.

DESCRIPTION OF THE EMBODIMENTS

One example of an embodiment in which the disclosure is applied isdescribed below. Furthermore, a size and a ratio of each member in thefollowing diagrams are used for convenience of description and are notlimited hereto. Additionally, in this application, with regard to arecitation of “arbitrary number A to arbitrary number B”, the number Ais included as a lower limit value and the number B is included as anupper limit value in the range. In addition, “sheet” in this applicationincludes not only the “sheet” defined in JIS but also “film”. Inaddition, numerical values specified in this application are valuesobtained by the method disclosed in the embodiment or examples.

An electromagnetic wave shielding sheet 10 according to the disclosureincludes a lamination body in which a conductive adhesive layer 1, ametal layer 2, and a protection layer 3 are laminated in this order (seeFIG. 1). In the electromagnetic wave shielding sheet 10, the conductiveadhesive layer 1 can be arranged on a component (not shown) and bondedto the component by a bonding treatment. The bonding treatment may beany treatment as long as the conductive adhesive layer 1 can be bondedto the component, for example, a heat treatment or a thermocompressionbonding treatment can be suitably used. The protection layer 3 plays arole of protecting the conductive adhesive layer 1 and the metal layer2, and is disposed closer to a surface-layer side than the metal layer2. The metal layer 2 is a layer sandwiched between the protection layer3 and the conductive adhesive layer 1 and mainly plays a role ofshielding electromagnetic waves. In a printed wiring board, the metallayer 2 plays a role of shielding electromagnetic noise generated fromsignal wirings and the like inside the component and shielding a signalfrom the outside.

The metal layer 2 has a plurality of openings 4, and an aperture ratioof the openings 4 is 0.1%-20%. In addition, a tensile breaking strengthof the electromagnetic wave shielding sheet 10 according to theembodiment is 10 N/20 mm-80 N/20 mm.

The openings 4 are also places where the protection layer 3 and theconductive adhesive layer 1 are adhered and play a role of improvingsolder reflow resistance. In addition, by setting the aperture ratiowhich is calculated from areas of the openings 4 and non-openings to0.1%-20%, both the solder reflow resistance and a high electromagneticwave shielding property can be achieved. Moreover, by setting thetensile breaking strength of the electromagnetic wave shielding sheet 10to 10 N/20 mm-80 N/20 mm, crack resistance can be improved, anddeterioration of an electromagnetic wave shielding property can besuppressed.

The electromagnetic wave shielding sheet and the printed wiring boardaccording to the embodiment are specifically described below. At first,the metal layer, the conductive adhesive layer, and the protection layerwhich configure the electromagnetic wave shielding sheet according tothe embodiment are specifically described.

(Metal Layer)

The metal layer of the disclosure has a plurality of openings, and theaperture ratio of the openings is 0.1%-20%. By setting the apertureratio of the openings to this range, a high electromagnetic waveshielding property can be maintained, and both the solder reflowresistance and the crack resistance can be achieved. The aperture ratiocan be adjusted from an area and the number of the openings. Inaddition, the aperture ratio is obtained by expression (1) below.

aperture ratio (%)=opening area per unit area/(opening area per unitarea+non-opening area per unit area)×100  expression (1)

A lower limit of the aperture ratio is more preferably 0.3% and furtherpreferably 0.5%. An upper limit of the aperture ratio is more preferably15% and further preferably 6.5%.

By setting the aperture ratio to the range of 0.1%-20%, the solderreflow resistance, the high electromagnetic wave shielding property andthe connection reliability of small opening via can be kept.

Measurement of the aperture ratio can be performed, for example, byusing an image which is obtained by magnifying the metal layer 500 timesto 5000 times with a laser microscope and a scanning electron microscope(SEM) perpendicular to a plane direction to binarize the openings andthe non-openings, and taking the number of pixels of binarized colorsper unit area as the respective areas.

In addition, with regard to the openings of the metal layer of thedisclosure, an average value of a circularity factor of the openingsobtained by expression (2) may be 0.5 or more.

circularity factor=(area×4π)/(perimeter)²  expression (2)

Herein, the perimeter is a length of an outer circumference when animage which is obtained by observing the metal layer by any one of anoptical microscope, a laser microscope and an electron microscope isread, a plane of the openings becomes a direction perpendicular to anobservation viewpoint, the opening capable of being entirely confirmedis extracted, and the extracted opening is projected two-dimensionally.The area is a breadth of a region defined by the outer circumferencewhen the extracted opening is projected two-dimensionally.

As described above, the metal layer of the disclosure has a plurality ofopenings. Preferably, there are openings on the entire surface of themetal layer. The openings play a role of releasing the volatilecomponent included in a polyimide film or a cover-lay adhesive of theprinted wiring board to the outside when the printed wiring board issubjected to a heating treatment such as solder reflow or the like.Accordingly, poor appearance and decrease of the connection reliabilitycaused by interfacial peeling of the cover-lay adhesive and theelectromagnetic wave shielding sheet can be suppressed. In addition, asshown in the schematic cross-section view of FIG. 1, by the adhesionbetween the protection layer and the conductive adhesive layer insidethe openings internal, interfacial adhesive forces between theprotection layer/the metal layer/the conductive adhesive layer arefurther improved, and the solder reflow resistance is further improved.

The opening has a circular shape, and the average value of thecircularity factor obtained by expression (2) is preferably 0.5 or more.By setting the average value of the circularity factor to 0.5 or more,even if a tension in the plane direction is applied to the metal layer,cracks are hard to generate in peripheral walls of the openings and thusthe crack resistance can be improved. The average value of thecircularity factor is more preferably 0.7 or more.

In addition, by setting the circularity factor in the above range, theelectromagnetic wave shielding property over time under high temperatureand high humidity can be improved. The reason is considered to be thatif the value of the circularity factor is small, the openings aredistorted, and a void portion which is not completely filled in theopenings is generated when the conductive layer and the protection layerare bonded. When the electromagnetic wave shielding sheet is stored fora long time under a high temperature and high humidity environment,moisture intrudes into the void, and rust is generated around theopenings. Accordingly, the conductivity partially deteriorates, and theelectromagnetic wave shielding property also deteriorates.

By the circularity factor in the above expression (2), a concave-convexdegree (an unevenness degree) of an outer edge of the opening can begrasped. A perfect circle has a circularity factor of 1, and thecircularity factor decreases with an increase of a concave-convex shape.That is, the circularity factor is 0 or more and 1 or less. With regardto the circularity factor in this disclosure, analysis software ofMac-View Ver.4 (Mountech Corporation) is used, the image of (about 500times to 10,000 times) the openings of the metal layer is read by alaser microscope or an electron microscope, and about 20 openings areselected in a manual recognition mode. Under a setting in which particlereference data is a projection area circle equivalent diameter anddistribution is volume distribution, the circularity factor iscalculated, and an average value of the 20 openings is obtained. Withregard to the area in the above expression (2), an area inside the linethat forms the outer circumference during the two-dimensional projectionis set as an area of a plane surface, and a length of the outercircumference of the opening when the plane surface is projectedtwo-dimensionally is set as a length of the perimeter.

The area of each opening is preferably 0.7 μm²-5000 μm², more preferably10 μm²-4000 μm², and further preferably 20 μm²-2000 μm². By setting theopening area to 0.7 μm² or more, the protection layer and the conductiveadhesive layer are excellently adhered, and the solder reflow resistanceis more excellent. By setting the opening area to 5000 μm² or less, thehigh electromagnetic wave shielding property can be excellent, and thusthe above range is preferable.

The number of the openings is preferably 100/cm²-200000/cm², morepreferably 1000/cm²-150000/cm², and further preferably1000/cm²-20000/cm². By setting the number of the openings to 100/cm² ormore, the volatile component is efficiently released to the outsideeasily, and thus the solder reflow resistance can be further improved.By setting the number of the openings to 200000/cm² or less, the highelectromagnetic wave shielding property can be ensured, and thus theabove range is preferable.

The thickness of the metal layer is preferably 0.5 μm-5 μm. Thethickness of the metal layer is more preferably 1.0 μm-4.5 μm, andfurther preferably 1 μm-4 μm. By the thickness of the metal layer beingin the range of 0.5 μm-5 μm, a balance between the high electromagneticwave shielding property and the crack resistance can be established.

The metal layer can use, for example, a metal foil, a metal depositionfilm, and a metal plating film.

Preferably, a metal used in the metal foil is, for example, a conductivemetal such as aluminum, copper, silver, gold and the like; in terms ofthe electromagnetic wave shielding property and cost, copper, silver,aluminum are more preferable and copper is further preferable. Withregard to the copper, for example, a rolled copper foil or anelectrolytic copper foil is preferably used, and the electrolytic copperfoil is more preferable. If the electrolytic copper foil is used, thethickness of the metal layer can be thinner. In addition, the metal foilmay also be formed by plating.

With regard to a metal used in the metal deposition film and the metalplating film, for example, aluminum, copper, silver, and gold arepreferable, and copper and silver are more preferable. The metal layeris preferably the deposition film in terms of film thinning. The metalfoil is preferable in terms of the electromagnetic wave shieldingproperty.

<Manufacturing Method of Metal Layer>

With regard to a manufacturing method of the metal layer having theopenings, a conventionally well-known method can be applied, a method(i) in which a pattern resist layer is formed on the metal foil and themetal foil is etched to form the openings, a method (ii) in whichconductive paste is printed in a predefined pattern by screen printing,a method (iii) in which an anchor agent is screen printed in apredefined pattern and a metal is plated only on the anchor agentprinted surface, a manufacturing method (iv) recited in JapaneseLaid-Open No. 2015-63730, and the like can be applied.

That is, a water-soluble or solvent-soluble ink is pattern printed on asupport body, a metal deposition film is formed on the surface and thepattern is removed. A release layer is formed on the surface and theopenings with carriers can be obtained by electrolytic plating. Amongthese, the opening formation method (i) in which the pattern resistlayer is formed and the metal foil is etched is preferable becauseshapes of the openings can be precisely controlled. However, themanufacturing method of the metal layer is not limited to the etchingmethod (i) as long as the shapes of the openings are controlled in othermethod.

(Conductive Adhesive Layer)

The conductive adhesive layer can be formed using a conductive resincomposition. The conductive resin composition includes a thermosettingresin and a conductive filler. The conductive adhesive layer may be anisotropic conductive adhesive layer or an anisotropic conductiveadhesive layer. The isotropic conductive adhesive layer has conductivityin a vertical direction and a horizontal direction in a state that theelectromagnetic wave shielding sheet is arranged horizontally. Inaddition, the anisotropic conductive adhesive layer only hasconductivity in the vertical direction in the state that theelectromagnetic wave shielding sheet is arranged horizontally.

The conductive adhesive layer may be an isotropic conductive adhesivelayer or an anisotropic conductive adhesive layer, and in the case ofthe anisotropic conductive adhesive layer, the cost can be reduced andthus the anisotropic conductive adhesive layer is preferable.

<Thermosetting Resin>

The thermosetting resin is a resin which has a plurality of functionalgroups capable of reacting with a curing agent. The functional group maybe, for example, a hydroxyl group, a phenolic hydroxyl group, amethoxymethyl group, a carboxyl group, an amino group, an epoxy group,an oxetanyl group, an oxazoline group, an oxazine groups, an aziridinegroup, a thiol group, an isocyanate group, a blocked isocyanate group, ablocked carboxyl group, a silanol groups or the like. The thermosettingresin may be, for example, a well-known resin such as an acrylic resin,a maleic acid resin, a polybutadiene resin, a polyester resin, apolyurethane resin, a polyurethane urea resin, an epoxy resin, anoxetane resin, a phenoxy resin, a polyimide resin, a polyamide resin, apolyamide imide resin, a phenolic resin, an alkyd resin, an amino resin,a polylactic acid resin, an oxazoline resin, a benzoxazine resin, asilicone resin, a fluorine resin or the like.

The thermosetting resin can be used alone or in combination of two ormore kinds.

Among these resins, in terms of solder reflow resistance, thepolyurethane resin, the polyurethane urea resin, the polyester resin,the epoxy resin, the phenoxy resin, the polyimide resin, the polyamideresin, and the polyamide imide resin are preferable.

An acid value of the thermosetting resin is preferably 1 mgKOH/g-50mgKOH/g and more preferably 3 mgKOH/g-30 mgKOH/g. By setting the acidvalue to 1 mgKOH/g-50 mgKOH/g, the solder reflow resistance is furtherimproved.

The content of the thermosetting resin in solid matter of the conductiveadhesive layer is preferably 10 weight %-80 weight %, and morepreferably 15 weight %-80 weight %. By the content being in the aboveblending range, the solder reflow resistance and the crack resistancecan be improved.

<Curing Agent>

The curing agent has a plurality of functional groups capable ofreacting with the functional groups of the thermosetting resin. Thecuring agent may be, for example, a well-known compound such as an epoxycompound, an acid anhydride group containing compound, an isocyanatecompound, an aziridine compound, an amine compound, a phenolic compound,an organic metal compound or the like.

The curing agent can be used alone or in combination of two or morekinds.

Preferably, 7 parts by weight-50 parts by weight of each kind of thecuring agent are included with respect to 100 parts by weight of thethermosetting resin, 9 parts by weight-40 parts by weight are morepreferable, and 10 parts by weight-30 parts by weight are furtherpreferable.

<Conductive Filler>

The conductive filler has a function of imparting conductivity to theconductive adhesive layer. With regard to the conductive filler, as araw material, for example, a conductive metal such as gold, platinum,silver, copper, nickel and the like and an alloy thereof, and fineparticles of conductive polymers are preferable, and silver is morepreferable in terms of price and conductivity.

In addition, in terms of cost reduction, instead of the fine particle ofa single raw material, a composite fine particle which uses metal orresin as a core and has a covering layer covering a surface of the coreis preferable. Herein, preferably, the core is appropriately selectedfrom cheap nickel, silica, copper and an alloy thereof, and a resin. Thecovering layer is preferably a conductive metal or a conductive polymer.The conductive metal may be, for example, gold, platinum, silver,nickel, manganese, indium or the like, and an alloy thereof. Inaddition, the conductive polymer may be polyaniline, polyacetylene orthe like. Among these materials, silver is preferable in terms of priceand conductivity.

The shape of the conductive filler is not limited as long as thedesirable conductivity is obtained. Specifically, for example, a sphereshape, a flake shape, a leaf shape, a branch shape, a plate shape, aneedle shape, a rod shape, and a grape shape are preferable. Inaddition, two types of conductive fillers of different shapes may bemixed.

The conductive filler can be used alone or in combination of two or morekinds.

An average particle diameter of the conductive filler is a D₅₀ averageparticle diameter, and in terms of sufficiently ensuring theconductivity, the average particle diameter of the conductive filler ispreferably 2 μm or more, more preferably 5 μm or more, and furtherpreferably 7 μm or more. On the other hand, from a viewpoint ofachieving both the conductivity and thinness of the conductive adhesivelayer, the average particle diameter of the conductive filler ispreferably 30 μm or less, more preferably 20 μm or less, and furtherpreferably 15 μm or less. The D₅₀ average particle diameter can beobtained by a laser diffraction-scattering particle size distributionmeasurement device or the like.

The content of the conductive filler in the conductive adhesive layer ispreferably 35 weight %-90 weight %, more preferably 39 weight %-85weight %, and further preferably 40 weight %-80 weight %. By setting thecontent to 35 weight % or more, the connection reliability of the smallopening via is improved. On the other hand, by setting the content to 90weight % or less, the adhesive force of the conductive adhesive layer isincreases and thus the solder reflow resistance is improved.

The conductive resin composition can be blended with other optionalcomponents such as a silane coupling agent, an antirust agent, areducing agent, an oxidation inhibitor, a pigment, a dye, a tackifyingresin, a plasticizing agent, an ultraviolet absorbing agent, anantifoaming agent, a leveling adjustment agent, a filler, aflame-retardant agent and the like.

The conductive resin composition can be obtained by mixing and stirringthe materials described above. The stirring can use, for example, awell-known stirring device such as a disperse mat, a homogenizer or thelike.

A well-known method can be used for manufacturing the conductiveadhesive layer. For example, a method in which the conductive adhesivelayer is formed by coating and drying the conductive resin compositionon a peelable sheet can be used; alternatively, the conductive adhesivelayer can also be formed by extruding the conductive resin compositioninto a sheet shape using an extrusion molding machine such as T-die.

The coating method includes, for example, a well-known coating methodsuch as a gravure coating method, a kiss coating method, a die coatingmethod, a lip coating method, a comma coating method, a blade method, aroll coating method, a knife coating method, a spray coating method, abar coating method, a spin coating method, a dip coating method or thelike. Preferably, a drying process is carried out during coating. Forexample, a well-known dry device such as a hot air dryer, an infraredheater or the like can be used for the drying process.

The thickness of the conductive adhesive layer is preferably 2 μm-30 μm,more preferably 3 μm-15 μm and further preferably 4 μm-9 μm. By thethickness being in the range of 2 μm-30 μm, the solder reflow resistanceand the connection reliability of the small opening via can be improved.

(Protection Layer)

The protection layer can be formed using a conventionally well-knownresin composition.

The resin composition can include the thermosetting resin and the curingagent described in the conductive resin composition and the aboveoptional components as required. Furthermore, the thermosetting resinsand the curing agents used in the protection layer and the conductiveadhesive layer may be the same or different.

The resin composition can be obtained in the same method as theconductive resin composition.

In addition, for the protection layer, a film which is obtained bymolding an insulation resin such as polyester, polycarbonate, polyimide,polyphenylene sulfide or the like can also be used.

The thickness of the protection layer is usually about 2 μm-12 μm.

(Electromagnetic Wave Shielding Sheet)

The electromagnetic wave shielding sheet of the disclosure at leastincludes the protection layer, the metal layer having the openings, andthe conductive adhesive layer.

The electromagnetic wave shielding sheet of the disclosure includes themetal layer having a plurality of openings and the aperture ratio of themetal layer is 0.1%-20%, and thus crosstalk and the like can be furthersuppressed by a wiring board for transmitting a particularly highfrequency (for example, from 100 MHz to 50 GHz) signal.

In addition, the electromagnetic wave shielding sheet of the disclosurehas a tensile breaking strength of 10 N/20 mm-80 N/20 mm. By setting thetensile breaking strength to the above range, the crack resistance canbe improved, and the deterioration of the electromagnetic wave shieldingproperty can be suppressed. A more preferable range of the tensilebreaking strength is 20 N/20 mm-70 N/20 mm, and a further preferablerange is 30 N/20 mm-65 N/20 mm.

In the disclosure, the tensile breaking strength of the electromagneticwave shielding sheet can be controlled using, for example, the apertureratio of the openings in the metal layer, the thickness, and thecircularity factor of the openings.

Specifically, the smaller the aperture ratio of the metal layer is, thehigher the tensile breaking strength becomes, and the higher theaperture ratio of the metal layer is, the lower the tensile breakingstrength becomes. The reason is that, when the aperture ratio of themetal layer is lower, a contacting area of the metal layer and theconductive adhesive layer increases, and the metal layer and theconductive adhesive layer are hard to be peeled.

In addition, the thinner the thickness of the metal layer is, the lowerthe tensile breaking strength becomes, and the thicker the thickness ofthe metal layer is, the higher the tensile breaking strength becomes.The reason is that, when the thickness of the metal layer is thicker,the strength of the metal layer increases.

In addition, the lower the circularity factor of the openings of themetal layer is, the lower the tensile breaking strength becomes, and thehigher the circularity factor of the openings is, the higher the tensilebreaking strength becomes. The reason is that, when the circularityfactor of the openings is higher, the shape of the openings is closer toa circle, and when tensile deformation stress is applied to theelectromagnetic wave shielding sheet, crack initiation points decrease.

Besides, in the disclosure, in addition to the factors described above,the tensile breaking strength of the electromagnetic wave shieldingsheet may also be controlled by compositions or configurations of theprotection layer and the conductive adhesive layer. For example, byincreasing the content of the curing agent in the conductive adhesivelayer, a curing degree of a curing system consisting of the resin andthe curing agent can be increased, the strength of the conductiveadhesive layer is increased, and the tensile breaking strength of theelectromagnetic wave shielding sheet can be increased.

Furthermore, with regard to a method for increasing the strength of theconductive adhesive layer, in addition to the above-described method,change of the type of the resin or the type of the curing agent and amethod of adding a filler or the like can also be used. In addition,with regard to the protection layer, the strength can also be controlledby the same method as the above-described conductive adhesive layer.

In addition, the electromagnetic wave shielding sheet of the disclosurecan have an excellent electromagnetic wave shielding property that whenthe electromagnetic wave shielding sheet is bonded to a wiring boardhaving a coplanar circuit, signal wirings of a wiring board having amicrostrip line circuit and the protection layer of the electromagneticwave shielding sheet are laminated, and a sine wave of 10 GHz flowsthrough the signal wirings of the microstrip line circuit, the crosstalkof the coplanar circuit is less than −45 dB.

Specifically, for example, the electromagnetic wave shielding propertycan be evaluated as described below.

First, the coplanar circuit is prepared.

The coplanar circuit is one of plane transmission circuits in whichsignal wirings are printed on one side of an insulation base materialsuch as a polyimide film or the like, and in the disclosure, thecoplanar circuit is a circuit in which a ground wiring is formed inparallel in a form of clamping two signal wirings on the polyimide film.Furthermore, for the above coplanar circuit, grounding patterns forground-connection are arranged on an opposite surface via through holes.

A conductive adhesive layer surface of the electromagnetic waveshielding sheet is bonded to an insulation base material surfaceopposite to the signal wirings of the coplanar circuit, and anelectromagnetic wave shielding layer is formed by thermocompressionbonding. At this time, the electromagnetic wave shielding sheet isconducted with the partially exposed ground patterns.

Next, signal wirings of a printed wiring board prepared separately andhaving a microstrip line circuit is arranged on a protection layersurface of the electromagnetic wave shielding layer formed on thecoplanar circuit, and a test piece for measurement is obtained. Anetwork analyzer is connected to the coplanar circuit and the microstripline circuit of the test piece, when a sine wave of 10 MHz to 20 GHzflows through the signal wirings of the microstrip line circuit, thecrosstalk in the coplanar circuit is measured, and the electromagneticwave shielding property can be evaluated.

Furthermore, a polyimide cover-lay film with adhesive is attached to theabove coplanar circuit and the above microstrip line circuit, but onepart of the circuits is exposed for connecting a probe of the networkanalyzer.

In the disclosure, the crosstalk of the coplanar circuit when the sinewave of 10 GHz flows through the signal wirings of the microstrip linecircuit is preferably less than −45 dB, more preferably less than −50dB, and further preferably less than −55 dB. By the crosstalk being lessthan −45 dB, the high electromagnetic wave shielding property can beobtained.

The electromagnetic wave shielding sheet according to the disclosure hasa plurality of openings, and the aperture ratio of the opening is0.1%-20%. In addition, the tensile breaking strength of theelectromagnetic wave shielding sheet is 10 N/20 mm-80 N/20 mm.Therefore, the solder reflow resistance and the crack resistance areexcellent. As a result, a resistance value in the plane direction of themetal layer seldom changes, the high electromagnetic wave shieldingproperty can be stably maintained, and failure of electronic componentsis reduced when the electromagnetic wave shielding sheet is mounted intoa narrow case.

The thermosetting resin and the curing agent included in the conductiveadhesive layer are present in an uncured state (B stage) and are curedby thermal pressing with the wiring board (C stage), and thereby theelectromagnetic wave shielding sheet can obtain a desirable adhesivestrength. Furthermore, the uncured state includes a half-cured state inwhich part of the curing agent is cured.

The peelable sheet is a sheet obtained by carrying out a well-known peeltreatment to a base material such as paper, plastic or the like.

Furthermore, in order to prevent adhesion of a foreign matter, theelectromagnetic wave shielding sheet is generally stored in a state thatthe peelable sheet is attached to the conductive adhesive layer and theprotection layer.

The electromagnetic wave shielding sheet can include other functionallayers in addition to the protection layer, the metal layer, and theconductive adhesive layer. The other functional layer is a layer havinga function such as a hard coat property, a water vapor barrier property,an oxygen barrier property, a thermal conductivity, a low dielectricconstant property, a high dielectric constant property, a heat-resistingproperty or the like.

The electromagnetic wave shielding sheet of the disclosure can be usedin various applications in which the shielding of electromagnetic wavesis required. For example, the electromagnetic wave shielding sheet canbe used in, in addition to a flexible printed wiring board, a rigidprinted wiring board, a COF, a TAB, a flexible connector, a liquidcrystal display, a touch panel or the like. In addition, theelectromagnetic wave shielding sheet can also be used as a computercase, building materials such as walls and window glass of a building, amember of a vehicle, a ship, an aircraft or the like for shieldingelectromagnetic waves.

<Manufacturing Method of Electromagnetic Wave Shielding Sheet>

In the manufacturing of the electromagnetic wave shielding sheet, awell-known method can be used as a method for laminating the conductiveadhesive layer and the metal layer.

For example, the methods (i)-(v) may be exemplified.

(i) A method in which a conductive adhesive layer is formed on apeelable sheet, the conductive adhesive layer is overlapped andlaminated on an electrolytic copper foil surface side of an electrolyticcopper foil having openings with copper carrier, and subsequently thecopper carrier is peeled. Then, a surface from which the copper carrieris peeled and a protection layer separately formed on a peelable sheetare overlapped and laminated. (ii) A method in which a protection layeris formed on a peelable sheet, the protection layer is overlapped andlaminated on an electrolytic copper foil surface side of an electrolyticcopper foil having openings with copper carrier, and subsequently thecopper carrier is peeled. Then, a surface from which the copper carrieris peeled and a conductive adhesive layer separately formed on apeelable sheet are overlapped and laminated. (iii) A method in which aresin composition is coated on an electrolytic copper foil surface sideof an electrolytic copper foil having openings with copper carrier toform a protection layer, and a peelable sheet is attached. Thereafter,the copper carrier is peeled, and a conductive adhesive layer separatelyformed on a peelable sheet is overlapped and laminated. (iv) A method inwhich a conductive adhesive layer is formed on a peelable sheet, theconductive adhesive layer is overlapped and laminated on an electrolyticcopper foil surface side of an electrolytic copper foil with coppercarrier, and subsequently the copper carrier is peeled. Then, after asurface from which the copper carrier is peeled and a protection layerseparately formed on a peelable sheet are overlapped and laminated,openings are formed in the electromagnetic wave shielding sheet with aneedle-shape jig. (v) A method in which after a protection layer formedon a peelable sheet is overlapped and laminated on an electrolyticcopper foil surface side of an electrolytic copper foil having openingswith copper carrier, the copper carrier is peeled. Then, a conductiveadhesive layer is formed on a surface from which the copper carrier ispeeled.

(Printed Wiring Board)

The printed wiring board of the disclosure includes the electromagneticwave shielding sheet, a cover coat layer, and a wiring board which has acircuit pattern having a signal wiring and a ground wiring and aninsulation base material. The electromagnetic wave shielding sheet isconfigured by the protection layer, the metal layer, and the conductiveadhesive layer, wherein the metal layer has a plurality of openings, theaperture ratio of the openings is 0.1%-20%, and the tensile breakingstrength of the electromagnetic wave shielding sheet is 10 N/20 mm-80N/20 mm.

In the printed wiring board of the disclosure, the electromagnetic waveshielding layer is formed by performing thermocompression bonding on theelectromagnetic wave shielding sheet which is configured by theprotection layer, the metal layer, and the conductive adhesive layer.The metal layer has a plurality of openings, the aperture ratio of theopenings is 0.1%-20%, and the tensile breaking strength of theelectromagnetic wave shielding sheet is 10 N/20 mm-80 N/20 mm.

The wiring board has the circuit pattern having the signal wiring andthe ground wiring on the surface of the insulation base material. On thewiring board, the cover coat layer which insulates and protects thesignal wiring and the ground wiring and has a via on at least part ofthe ground wiring is formed. After the conductive adhesive layer surfaceof the electromagnetic wave shielding sheet is arranged on the covercoat layer, the electromagnetic wave shielding sheet is bonded bythermocompression, the conductive adhesive layer is allowed to flow intothe via and be adhered to the ground wiring, and thereby the printedwiring board can be manufactured.

One example of the printed wiring board 7 of the disclosure is describedwith reference to FIG. 1.

The electromagnetic wave shielding sheet 10 includes the protectionlayer 3, the metal layer 2 having the plurality of openings, and theconductive adhesive layer 1.

The cover coat layer 8 is an insulating material which covers the signalwiring 6 of the wiring board and protects the signal wiring 6 from theexternal environment.

The cover coat layer 8 is preferably a polyimide film with aheat-curable adhesive, a heat-curable or ultraviolet-curable solderresist, or a photosensitive cover-lay film, and more preferably thephotosensitive cover-lay film for micro-fabrication. In addition, thecover coat layer generally uses a well-known resin such as polyimide orthe like having heat-resisting property and flexibility. The thicknessof the cover coat layer 8 is usually about 10 μm-100 μm.

The circuit pattern includes the ground wiring 5 for grounding, and thesignal wiring 6 for sending electric signals to the electroniccomponent. Both the ground wiring 5 and the signal wiring 6 are usuallyformed by etching a copper foil. The thickness of the circuit pattern isusually about 1 μm-50 μm.

The insulation base material 9 is a support body of the circuit patternand is preferably a bendable plastic such as polyester, polycarbonate,polyimide, polyphenylene sulfide, a liquid crystal polymer or the like,and more preferably the liquid crystal polymer and polyimide. Inparticular, considering the application of the printed circuit boardtransmitting high frequency signals, the liquid crystal polymer having alow relative dielectric constant and a low dielectric loss tangent isfurther preferable.

When the wiring board is a rigid wiring board, a configuration materialof the insulation base material is preferably glass epoxy. The wiringboard can obtain a high heat-resisting property by including theseinsulation base materials.

The thermal pressing of the electromagnetic wave shielding sheet 10 andthe wiring board is generally carried out under conditions of atemperature of about 150° C.-190° C., a pressure of about 1 MPa-3 MPa,and a time of about 1 minute −60 minutes. By the thermal pressing, theconductive adhesive layer 1 and the cover coat layer 8 are in closecontact, the conductive adhesive layer 1 flows to fill the via 11 formedin the cover coat layer 8 and thereby conductivity with the groundwiring 5 is established. By the thermal pressing, the thermosettingresin is reacted and cured.

Furthermore, in order to facilitate the curing, post-cure may also becarried out for 30 minutes-90 minutes at 150° C.-190° C. after thethermal pressing. Furthermore, the electromagnetic wave shielding sheetmay be called the electromagnetic wave shielding layer after the thermalpressing.

An opening area of the via 11 is preferably 0.008 mm² or more and 0.8mm² or less, more preferably 0.3 mm² or less, and particular preferably0.03 mm² or less. By setting the opening area to the above range, whilethe ground-connection reliability is ensured and the highelectromagnetic wave shielding property is maintained, a region of theground wiring can be narrowed, and miniaturization of the printed wiringboard can be achieved.

The shape of the via is not particularly limited, and any one of circle,square, rectangle, triangle, irregular shape and the like can be useddepending on the application.

It is preferable that the electromagnetic wave shielding layer is formedon both surfaces of the wiring board in that leakage of theelectromagnetic wave can be more effectively suppressed. Theelectromagnetic wave shielding sheet of the disclosure is configured bythe protection layer, the metal layer, and the conductive adhesivelayer, wherein the metal layer has a plurality of openings, and theaperture ratio of the openings is 0.1%-20%. In addition, in the openingsof the metal layer, the average value of the circularity factor obtainedby expression (2) may be 0.5 or more. Because the electromagnetic waveshielding sheet of the disclosure has the aforementioned configuration,after the electromagnetic wave shielding layer is formed on bothsurfaces of the wiring board, even when a reflow treatment is carriedout, no foaming occurs because internal residual gas is discharged tothe outside through the openings 4. In addition, the electromagneticwave shielding sheet 10 in the printed wiring board of the disclosurecan be used as a ground circuit in addition to shielding theelectromagnetic waves, and thereby part of the ground circuit isomitted, and the area of the printed wiring board is reduced, by whichcost reduction is possible and the printed wiring board can beincorporated in a narrow region within a housing.

In addition, the signal wiring is not particularly limited and can beused in both a single end consisting of one signal wiring and adifferential circuit consisting of two signal wirings, and thedifferential circuit is more preferable. On the other hand, when thereis a restriction on a circuit pattern area of the printed wiring board,and the ground circuits are hard to be formed in parallel, a groundcircuit is not arranged beside the signal circuit, and theelectromagnetic wave shielding sheet can also be used as a groundcircuit to provide a printed wiring board structure having a ground in athickness direction.

The printed wiring board of the disclosure is preferably included in(mounted to) an electronic machine such as a notebook PC, a mobilephone, a smart phone, a tablet terminal or the like in addition to aliquid crystal display, a touch panel or the like.

EXAMPLES

The disclosure is more specifically described below by examples, but thedisclosure is not limited to the examples below. In addition, a term“parts” in the examples means “parts by weight” and “%” means “weight%”.

Furthermore, measurement of the acid value, an weight average molecularweight (Mw) and a glass transition temperature (Tg) of the resin, theaverage particle diameter of the conductive filler, the circularityfactor of the openings of the metal layer, and the tensile breakingstrength of the electromagnetic wave shielding sheet are carried out bymethods below.

<Measurement of Acid Value of Resin>

The acid value is measured in accordance with JIS K0070. Approximately 1g of a sample is precisely weighed into a stoppered Erlenmeyer flask,and 100 ml of a tetrahydrofuran/ethanol (volume ratio:tetrahydrofuran/ethanol=2/1) mixture is added to dissolve the sample. Aphenolphthalein test liquid is added as an indicator into the abovesolution, titration with 0.1N of alcoholic potassium hydroxide solutionis carried out, and an end point is arrived when the indicator maintainspink for 30 seconds. The acid value is obtained by an expression below(unit: mgKOH/g).

acid value (mgKOH/g)=(5.611×a×F)/S

Herein,

S: a collection amount (g) of the sample

a: a consumption amount (ml) of the 0.1N alcoholic potassium hydroxidesolution

F: titer of the 0.1N alcoholic potassium hydroxide solution

<Measurement of Weight Average Molecular Weight (Mw) of Resin>

In the measurement of the weight average molecular weight (Mw), GPC (gelpermeation chromatography) “HPC-8020” made by TOSOH Corporation is used.GPC is a liquid chromatography which separates and quantifies asubstance dissolved in a solvent (THF; tetrahydrofuran) based on adifference in molecular size of the substance. The measurement in thedisclosure is carried out using two “LF-604” (Showa Denko K.K.: GPCcolumn for rapid analysis: a size of 6 mmID×150 mm) connected in seriesas a column under conditions of a flow rate of 0.6 ml/min and a columntemperature of 40° C., and the weight average molecular weight (Mw) isdetermined in terms of polystyrene.

<Glass Transition Temperature (Tg) of Resin>

The measurement of Tg is carried out by differential scanningcalorimetry (“DSC-1” made by Mettler Toledo International Inc.).

<Measure of Average Particle Diameter of Conductive Filler>

The D₅₀ average particle diameter is a value which is obtained bymeasuring a conductive filler using a laser diffraction-scatteringparticle size distribution measurement device LS13320 (Beckman Coulter,Inc.) by a tornado dry powder sample module, and is the particlediameter when a cumulative value in a particle size cumulativedistribution is 50%. Furthermore, a refractive index is set to 1.6.

<Measurement of Circularity Factor of Openings>

A plane image of the metal layer is obtained at a magnification of 2000times-5000 times using a reflection electron microscope JSM-IT100 (JapanElectron Optics Laboratory Corporation) in a manner that about 20openings of the metal layer are contained, and the plane image isanalyzed by the above method.

<Measurement of tensile breaking Strength>

Two pieces of the electromagnetic wave shielding sheets are prepared,release films on the conductive adhesive layer side of the respectiveelectromagnetic wave shielding sheets are peeled, and conductiveadhesive layer surfaces of each other are bonded by a hot roll laminatorto obtain a lamination body. After the lamination body is cut into asize of a width 20 mm×a length 60 mm, the peelable film on theprotection layer side is peeled off on both sides to obtain ameasurement sample. With regard to the measurement sample, a tensiletest (test speed 50 mm/min) is conducted under conditions of atemperature of 25° C. and a relative humidity of 50% using a small-sizeddesktop tester EZ-TEST (manufactured by Shimadzu Corporation). Thetensile breaking strength of the electromagnetic wave shielding sheet(N/20 mm) is calculated from a S-S curve (Stress-Strain curve) that isobtained.

Next, raw materials used in the examples are shown below.

(Raw Materials)

Conductive filler: composite fine particles (dendrite-like fineparticles coated with 10 parts by weight of silver with respect to 100parts by weight of copper of the core), average particle diameter D₅₀:11.0 μM, manufactured by Fukuda metal foil and powder Corporation

Thermosetting resin: a polyurethane urea resin with an acid value of 5mgKOH/g, a weight average molecular weight of 54, 000, and a Tg of −7°C. (manufactured by Toyochem Corporation)

Epoxy compound: “JER828” (Bisphenol A type epoxy resin, epoxyequivalent=189 g/eq) manufactured by Mitsubishi Chemical Corporation

Aziridine compound: “Chemitite PZ-33” manufactured by Nippon ShokubaiCo., Ltd.

<Manufacturing of Conductive Resin Composition 1>

In terms of solid content, 100 parts of the thermosetting resin, 52parts of the conductive filler, 10 parts of the epoxy compound, and 0.5part of the aziridine compound are fed into a container, a mixed solvent(toluene:isopropyl alcohol=2:1 (weight ratio)) is added so thatnon-volatile content concentration is 40%, and the mixture is stirredfor 10 minutes with a disperser to obtain the conductive resincomposition.

The conductive resin composition is coated onto the peelable sheet by abar coater so that the dry thickness is 10 μm and the conductive resincomposition is dried in an electric oven at 100° C. for 2 minutes, andthereby the conductive resin composition (the conductive adhesive layer)1 is obtained.

<Manufacturing of Conductive Resin Compositions 2-15>

Conductive resin compositions (conductive adhesive layers) 2-15 shown intable 1 and table 2 are manufactured by the same method as theconductive resin composition 1 except that an additive amount of theconductive filler and an additive amount of the epoxy compound arechanged.

TABLE 1 Conductive Conductive Conductive Conductive Conductive resinresin resin resin resin composition composition composition compositioncomposition 1 2 3 4 5 Thermosetting 100 100 100 100 100 resin Epoxy 1010 10 10 10 compound Aziridine 0.5 0.5 0.5 0.5 0.5 compound Conductive52 59.63 71 74 166 filler Conductive 32% 35% 39% 40% 60% filler content[mass %] Conductive Conductive Conductive Conductive Conductive resinresin resin resin resin composition composition composition compositioncomposition 6 7 8 9 10 Thermosetting 100 100 100 100 100 resin Epoxy 1010 10 10 10 compound Aziridine 0.5 0.5 0.5 0.5 0.5 compound Conductive205 257 331 993 1259 filler Conductive 65% 70% 75% 90% 92% fillercontent [mass %]

TABLE 2 Conductive resin Conductive resin Conductive resin Conductiveresin Conductive resin composition 11 composition 12 composition 13composition 14 composition 15 Thermosetting resin 100 100 100 100 100Epoxy compound 6.5 8.5 29.5 39.5 49.5 Aziridine compound 0.5 0.5 0.5 0.50.5 Conductive filler 72 73 87 94 100 Conductive filler 40% 40% 40% 40%40% content [mass %]

Example 1

In terms of solid content, 100 parts of the thermosetting resin, 10parts of the epoxy compound and 1 part of the aziridine compound areadded, and the mixture is stirred for 10 minutes with a disperser toobtain the resin composition. After the resultant resin composition iscoated on the copper foil 1 using a bar coater so that the dry thicknessis 5 μm and dried in an electric oven at 100° C. for 2 minutes, aslightly tacky peelable sheet is bonded to the protection layer.

Next, the copper carrier of the copper foil is peeled, and theconductive adhesive layer is bonded to the copper foil surface, andthereby the electromagnetic wave shielding sheet including “the peelablesheet/the protection layer/the copper foil/the conductive adhesivelayer/the peelable sheet” is obtained. The copper foil 1 and theconductive adhesive layer are boned by a heat laminator at a temperatureof 90° C. and a pressure of 3 kgf/cm².

Furthermore, the copper foil is a copper foil having a circularityfactor of opening, an aperture ratio and the like shown in table 3. Theopenings of the copper foil are formed by forming a pattern resist layeron the copper foil formed on the copper carrier via a peel layer andetching the copper foil.

Examples 2-33, Comparison Examples 1-3

Electromagnetic wave shielding sheets of the examples 2-33 andcomparison example 1-3 are obtained respectively in the same way as theexample 1 except that the conductive adhesive layer and a type of thecopper foil in the example 1 are changed.

Furthermore, the copper foils of the examples and the comparisonexamples are obtained in the same way as the example 1 by a method inwhich the pattern resist layer is formed on the copper foil formed onthe peel layer via the copper carrier, and the openings are formed byetching. Circularity factors and aperture ratios of the openings of thecopper foils and the like in the examples and the comparison examplesare shown in table 3-table 6.

Evaluations below are carried out using the resultant electromagneticwave shielding sheet. Results are shown in table 3-table 6.

<Solder Reflow Resistance>

The solder reflow resistance is evaluated according to whether or not anappearance changes after the electromagnetic wave shielding sheet and amolten solder are brought into contact. The appearance of anelectromagnetic wave shielding sheet with a high solder reflowresistance does not change, while foaming or peeling occurs in anelectromagnetic wave shielding sheet with a low solder reflowresistance.

First, a peelable sheet of a conductive adhesive layer of anelectromagnetic wave shielding sheet with a width of 25 mm and a lengthof 70 mm is peeled, and the exposed conductive adhesive layer and agold-plated surface of a gold-plated copper clad lamination board (goldplating 0.3 μm/nickel plating 1 μm/copper foil 18 μm/adhesive 20μm/polyimide film 25 μm) with a total thickness of 64 μm are pressedunder conditions of 150° C., 2.0 MPa, and 30 minutes and thermally curedto obtain a lamination body. The resultant lamination body is cut into asize of a width of 10 mm and a length of 65 mm to manufacture a sample.The resultant sample is left for 72 hours in an atmosphere of 40° C. and90% RH. Thereafter, the sample is floated on the molten solder with thepolyimide film surface down at 250° C. for 1 minutes, next the sample istaken out, an appearance thereof is visually observed, and whether ornot there is an abnormality such as foaming, floating, peeling or thelike is evaluated based on criteria below.

⊚: No change in appearance.

◯: Few small bubbles are observed.

Δ: Many small bubbles are observed.

ΔΔ: Small bubbles are observed on the entire surface of the sample.

x: Severe foaming or peeling is observed.

<Electromagnetic Wave Shielding Property>

The electromagnetic wave shielding property is evaluated by measuringthe crosstalk. The crosstalk is evaluated using samples for measurementbelow.

(Manufacturing of Wiring Board Having Coplanar Circuit)

FIG. 2 is a schematic plan view of a principal surface side of theflexible printed wiring board having coplanar circuit (hereinafter, alsoreferred to as the wiring board having coplanar circuit) 20 used in themeasurement, and FIG. 3 is a schematic plan view of a back surface side.First, a double-surface CCL “R-F775” (manufactured by PanasonicCorporation) in which a rolled copper foil with a thickness of 12 μm islaminated on both surfaces of the polyimide film 50 with a thickness of50 μm is prepared. Then, six through holes 51 (diameter 0.1 mm) arerespectively arranged in the vicinity of four rectangular corners.Furthermore, in the diagram, for convenience of illustration, only twothrough holes 51 are shown at each corner. Next, after an electro-lessplating treatment is carried out, an electrolytic plating treatment iscarried out to form a copper plating film 52 of 10 μm, and conductionbetween both principal surfaces is ensured via the through holes 51.Thereafter, as shown in FIG. 2, on the principal surface of thepolyimide film 50, the ground pattern (i) 55 is formed in a regionincluding two signal wirings 53 with a length of 10 cm, ground wirings54 parallel to the signal wirings 53 outside the signal wirings 53, andthe through holes 51 in the short direction of the polyimide film 50 andextended from the ground wirings 54.

Thereafter, the copper foil formed on the back surface of the polyimidefilm 50 is etched, and a back surface side ground pattern (ii) 56 shownin FIG. 3 is obtained in a place corresponding to the ground pattern (i)55. Check specifications of the appearances of the circuit and toleranceare in accordance with the JPCA standard (JPCA-DG02). Next, a cover coatlayer 8 “CISV1215 (manufactured by NIKKAN Industries Co., Ltd.)”configured by a polyimide film 8 a (thickness 12.5 μm) and a conductiveadhesive layer 8 b (thickness 15 μm) is attached to the principalsurface side of the polyimide film 50 (see FIG. 2). Furthermore, in FIG.2, the cover coat layer 8 is shown in a perspective view so that astructure of the signal wirings 53 and the like can be understood.Thereafter, the copper foil pattern exposed from the cover coat layer 8is nickel-plated (not shown) and then gold-plating (not shown) treatmentis carried out.

Next, as shown in FIG. 4, the electromagnetic wave shielding sheet 10including the lamination body of the conductive adhesive layer 1, themetal layer 2, and the protection layer 3 is prepared, and the peelablesheet (not shown) arranged on the conductive adhesive layer 1 of theelectromagnetic wave shielding sheet 10 is peeled. Then, the conductiveadhesive layer 1 of the electromagnetic wave shielding sheet 10 is takenas an inner side and pressed on an entire back surface side of thewiring board 20 having the coplanar circuit under conditions of 150° C.,2.0 MPa, and 30 minutes, and thereby the wiring board 20 having thecoplanar circuit with the electromagnetic wave shielding sheet isobtained. In FIG. 4, the back surface side ground pattern (ii) 56 isshown in a perspective view.

(Manufacturing of Wiring Board Having Microstrip Line Circuit)

A wiring board 30 having microstrip line circuit is manufacturedseparately as shown in FIG. 5 and FIG. 6. First, the double-surface CCL“R-F775” (manufactured by Panasonic Corporation) in which the rolledcopper foil with a thickness of 12 μm is laminated is prepared. Then,two signal wirings 35 with a length of 10 cm are formed on one surfaceby etching. Check specifications of the appearance of the circuit andtolerance are in accordance with the JPCA standard (JPCA-DG02). Next, acover-lay 31 “CISV1215 (manufactured by NIKKAN Industries Co., Ltd.)”configured by a polyimide film 31 a (thickness 12.5 μm) and a conductiveadhesive layer 31 b (thickness 15 μm) is attached to the signal wirings35 side (see FIG. 5). Furthermore, in FIG. 5, the cover-lay 31 is shownin a perspective view so that a structure of the signal wirings 53 andthe like can be understood. Thereafter, the signal wirings 35 exposedfrom the cover-lay 31 are nickel-plated (not shown) and thengold-plating (not shown) treatment is carried out. In addition, as shownin FIG. 6, a grounding layer 34 is arranged on a back surface side ofthe polyimide film 33.

(Manufacturing of Test Piece)

Next, the signal wiring 35 side of the wiring board 30 having themicrostrip line circuit and the electromagnetic wave shielding sheet 10side of the wiring board 20 having the coplanar circuit are laminated tobe in contact and are fixed by a fixture. A schematic cross-section viewof the lamination body is shown in FIG. 7 and FIG. 8. FIG. 7 isequivalent to a cross-section view of a cutoff portion along XI-XI ofFIG. 2, and FIG. 8 is equivalent to a cross-section view of a cutoffportion along XII-XII of FIG. 2.

A network analyzer E5071C (manufactured by Agilent Japan Ltd.) isconnected to the exposed signal wirings 35 of the wiring board 30 havingthe microstrip line circuit and the exposed signal wirings 53 of thewiring board 20 having the coplanar circuit. Then, sine waves of 10MHz-20 GHz are input to the signal wirings 35 of the wiring board 30having the microstrip line circuit, the crosstalk in the wiring board 20having the coplanar circuit at this time is measured, and an influenceof the electromagnetic wave shielding property is confirmed according tothe value of the crosstalk.

Furthermore, L/S (line/space) of the signal wiring 35 is appropriatelyadjusted so that a characteristic impedance is within ±10Ω. A width ofthe ground wiring 54 is set to 100 μm, and a distance between the groundwiring 54 and the signal wiring 53 is set to 1 mm.

The measured crosstalk is evaluated based on criteria below. Evaluateresults (electromagnetic wave shielding properties) are shown in table3-table 6.

⊚: Crosstalk in 10 GHz is less than −55 dB

◯: Crosstalk in 10 GHz is −55 dB or more and less than −50 dB

Δ: Crosstalk in 10 GHz is −50 dB or more and less than −45 dB

x: Crosstalk in 10 GHz is −45 dB or more

<Electromagnetic Wave Shielding Property Over Time Under HighTemperature and High Humidity>

After the wiring board 20 having the coplanar circuit with the withelectromagnetic wave shielding sheet is left for 500 hours under a hightemperature and high humidity environment of 85° C. and 85%, thecrosstalk in 10 GHz is measured. Furthermore, the measurement of thecrosstalk in 10 GHz is the same as the measurement of theelectromagnetic wave shielding property except that the wiring board 20is left under the high temperature and high humidity environment for 500hours.

⊚: Crosstalk in 10 GHz is less than −55 dB

◯: Crosstalk in 10 GHz is −55 dB or more and less than −50 dB

Δ: Crosstalk in 10 GHz is −50 dB or more and less than −45 dB

x: Crosstalk in 10 GHz is −45 dB or more

<Crack Resistance>

An electromagnetic wave shielding sheet with a width of 50 mm and alength of 50 mm is thermally pressed under conditions of 150° C., 5.0MPa, and 30 minutes without peeling the peelable sheet of the conductiveadhesive layer. Thereafter, the peelable sheet is peeled, and whether ornot there is a crack in the metal layer is confirmed from a conductiveadhesive side by an optical microscope.

Evaluation criteria are as follows.

⊚: No crack, an extremely good result.

◯: 1-5 crack places, a good result.

Δ: 6-10 crack places, no problem in practical use.

x: 11 or more crack places, unavailable in practical use.

<Connection Reliability to Small Opening Via>

As shown in FIGS. 9(1)-9(3), on a polyimide film 21 with a thickness of25 μm, a copper foil circuit 22A and a copper foil circuit 22B with athickness of 18 μm which are not electrically connected to each otherare formed. Next, a polyimide cover-lay with adhesive 23 which has acircular via 24 with a thickness of 37.5 μm and a diameter of 1.1 mm (avia area is 1.0 mm²) is laminated on the copper foil circuit 22A to forma flexible printed wiring board.

In addition, an electromagnetic wave shielding sheet 25 with a size of awidth of 20 mm and a length of 50 mm is prepared. Then, as shown inFIGS. 9(4)-9(6), a peelable sheet is peeled from the electromagneticwave shielding sheet 25, an exposed conductive adhesive layer 25 b ispressed on the flexible printed wiring board formed as described aboveunder conditions of 150° C., 2 MPa, and 30 minutes, and the conductiveadhesive layer 25 b and a protection layer 25 a of the electromagneticwave shielding sheet are cured.

Next, the peelable sheet on the protection layer 25 a side of the sampleis removed, and an initial connection resistance value between 22A-22Bshown in the plan view of FIG. 9(4) is measured using a BSP probe of“Loresta GP” manufactured by Mitsubishi Chemical Corporation.Furthermore, FIG. 9(2) is a cross-section view along D-D′ in FIG. 9(1),and FIG. 9(3) is a cross-section view along C-C′ in FIG. 9(1).Similarly, FIG. 9(5) is a cross-section view along D-D′ in FIG. 9(4),and FIG. 9(6) is a cross-section view along C-C′ in FIG. 9(4). A viadiameter is made with a scale of 0.1 mm from 1.1 mm (the via area is 1.0mm²) to 0.1 mm (the via area is 0.008 mm²), connection reliability testsare carried out respectively in the same way as described above, and asmallest via diameter under which the connection resistance value is 200mΩ, or less is confirmed.

⊚: The smallest via diameter is 0.2 mm (the via area is 0.03 mm²) orless, an extremely good result.

◯: The smallest via diameter is 0.3 mm (the via area is 0.07 mm²) ormore and 0.6 mm (the via area is 0.3 mm²) or less, a good result.

Δ: The smallest via diameter is 0.7 mm (the via area is 0.4 mm²) or moreand 1.0 mm (the via area is 0.8 mm²) or less, no problem in practicaluse.

x: The smallest via diameter is 1.1 mm (the via area is 1.0 mm²) or theconnection resistance value does not reach 200 mΩ, or lower, unavailablein practical use.

TABLE 3 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Protection layer Resin Resin Resin Resin Resin Resin compositioncomposition composition composition composition composition 1 1 1 1 1 1Metal Type of Copper foil Copper foil Copper foil Copper foil Copperfoil Copper foil layer metal 1 2 3 4 5 6 layer Diameter 20 20 20 20 2020 of openings [μm] Opening 314 314 314 314 314 314 area [μm²] Number350 600 1000 1500 3200 20000 of openings [per cm²] Aperture 0.1% 0.2%0.3% 0.5% 1.0% 6.3% ratio [%] Circularity 0.7 0.7 0.7 0.7 0.7 0.7 factorof openings Thickness 3 3 3 3 3 3 [μm] Conductive Conductive ConductiveConductive Conductive Conductive Conductive adhesive layer resin resinresin resin resin resin composition composition composition compositioncomposition composition 4 4 4 4 4 4 Tensile breaking 80 60 55 53 50 45strength of electromagnetic wave shielding sheet Solder reflow Δ ◯ ⊚ ⊚ ⊚⊚ resistance Electromagnetic ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ wave shielding propertyElectromagnetic ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ wave shielding property over time under hightemperature and high humidity Crack resistance ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ Connection ⊚⊚ ⊚ ⊚ ⊚ ⊚ reliability of small opening via Example 7 Example 8 Example 9Example 10 Example 11 Protection layer Resin Resin Resin Resin Resincomposition composition composition composition composition 1 1 1 1 1Metal Type of Copper foil Copper foil Copper foil Copper foil Copperfoil layer metal 7 8 9 10 11 layer Diameter 20 20 20 20 20 of openings[μm] Opening 314 314 314 314 314 area [μm²] Number 30000 35000 4500050000 62000 of openings [per cm²] Aperture 9.4% 11.0% 14.1% 15.7% 19.5%ratio [%] Circularity 0.7 0.7 0.7 0.7 0.7 factor of openings Thickness 33 3 3 3 [μm] Conductive Conductive Conductive Conductive ConductiveConductive adhesive layer resin resin resin resin resin compositioncomposition composition composition composition 4 4 4 4 4 Tensilebreaking 40 38 35 31 28 strength of electromagnetic wave shielding sheetSolder reflow ⊚ ⊚ ⊚ ⊚ ⊚ resistance Electromagnetic ⊚ ◯ ◯ Δ Δ waveshielding property Electromagnetic ◯ ◯ ◯ Δ Δ wave shielding propertyover time under high temperature and high humidity Crack resistance ⊚ ⊚⊚ ⊚ ◯ Connection ⊚ ⊚ ⊚ ⊚ ⊚ reliability of small opening via

TABLE 4 Example Example Example Example Example Example Example Example12 13 14 15 16 17 18 19 Protection layer Resin Resin Resin Resin ResinResin Resin Resin composition composition composition compositioncomposition composition composition composition 1 1 1 1 1 1 1 1 MetalType of Copper foil Copper foil Copper foil Copper foil Copper foilCopper foil Copper foil Copper foil layer metal 12 13 14 15 16 17 18 19layer Diameter 20 20 20 20 20 20 20 20 of opening [μm] Opening 314 314314 314 314 314 314 314 area [μm²] Number 3200 3200 3200 3200 3200 32003200 3200 of openings [per cm²] Aperture 1.0% 1.0% 1.0% 1.0% 1.0% 1.0%1.0% 1.0% ratio [%] Circularity 0.5 0.6 0.7 0.7 0.7 0.7 0.7 0.7 factorof openings Thickness 3 3 0.5 1 4 4.5 5 5.5 [μm] Conductive ConductiveConductive Conductive Conductive Conductive Conductive ConductiveConductive adhesive layer resin resin resin resin resin resin resinresin composition composition composition composition compositioncomposition composition composition 4 4 4 4 4 4 4 4 Tensile breaking 2030 13 30 57 68 79 80 strength of electromagnetic wave shielding sheetSolder reflow ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ resistance Electromagnetic ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ⊚wave shielding property Electromagnetic Δ ◯ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ wave shieldingproperty over time under high temperature and high humidity Crackresistance ◯ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ Δ Connection ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ reliability ofsmall opening via

TABLE 5 Example Example Example Example Example Example Example ExampleExample 20 21 22 23 24 25 26 27 28 Protection layer Resin Resin ResinResin Resin Resin Resin Resin Resin composition composition compositioncomposition composition composition composition composition composition1 1 1 1 1 1 1 1 1 Metal Type of Copper foil Copper foil Copper foilCopper foil Copper foil Copper foil Copper foil Copper foil Copper foillayer metal 6 6 6 6 6 6 6 6 6 layer Diameter 20 20 20 20 20 20 20 20 20of openings [μm] Opening 314 314 314 314 314 314 314 314 314 area [μm²]Number 3200 3200 3200 3200 3200 3200 3200 3200 3200 of openings [percm²] Aperture 1.0% 1.0% 1.0% 1.0% 1.0% 1.0% 1.0% 1.0% 1.0% ratio [%]Circularity 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 factor of openingsThickness 3 3 3 3 3 3 3 3 3 [μm] Conductive Conductive ConductiveConductive Conductive Conductive Conductive Conductive ConductiveConductive adhesive layer resin resin resin resin resin resin resinresin resin composition composition composition composition compositioncomposition composition composition composition 1 2 3 5 6 7 8 9 10Tensile breaking 50 50 50 50 50 50 50 50 50 strength of electromagneticwave shielding sheet Solder reflow ⊚ ⊚ ⊚ ⊚ ⊚ ◯ Δ Δ ΔΔ resistanceElectromagnetic ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ wave shielding propertyElectromagnetic ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ wave shielding property over timeunder high temperature and high humidity Crack resistance ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚⊚ ⊚ Connection Δ ◯ ◯ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ reliability of small opening via

TABLE 6 Comparison Comparison Comparison Example 29 Example 30 Example31 Example 32 Example 33 example 1 example 2 example 3 Protection layerResin Resin Resin Resin Resin Resin Resin Resin composition compositioncomposition composition composition composition composition composition1 1 1 1 1 1 1 1 Metal Type of Copper foil Copper foil Copper foil Copperfoil Copper foil Copper foil Copper foil Copper foil layer metal 6 6 6 66 20 21 22 layer Diameter 20 20 20 20 20 20 20 20 of openings [μm]Opening 314 314 314 314 314 314 314 314 area [μm²] Number 3200 3200 32003200 3200 90 68000 3200 of openings [per cm²] Aperture 1.0% 1.0% 1.0%1.0% 1.0% 0.03% 21.4% 1.0% ratio [%] Circularity 0.7 0.7 0.7 0.7 0.7 0.70.7 0.7 factor of openings Thickness 3 3 3 3 3 3 3 0.1 [μm] ConductiveConductive Conductive Conductive Conductive Conductive ConductiveConductive Conductive adhesive layer resin resin resin resin resin resinresin resin composition composition composition composition compositioncomposition composition composition 11 12 13 14 15 4 4 4 Tensilebreaking 13 26 60 68 76 90 19 5 strength of electromagnetic waveshielding sheet Solder reflow ⊚ ⊚ ⊚ ⊚ ⊚ X ⊚ ⊚ resistance Electromagnetic⊚ ⊚ ⊚ ⊚ ⊚ ⊚ X Δ wave shielding property Electromagnetic ◯ ⊚ ⊚ ⊚ ⊚ ⊚ X ⊚wave shielding property over time under high temperature and highhumidity Crack resistance Δ ◯ ⊚ ⊚ ⊚ ⊚ Δ X Connection ⊚ ⊚ ⊚ ◯ Δ ⊚ X ⊚reliability of small opening via

According to the disclosure, the electromagnetic wave shielding sheethaving excellent solder reflow resistance and crack resistance, capableof highly reliable ground-connection even with respect to the smallopening via, and having a high electromagnetic wave shielding propertyeven when used in the high frequency transmission circuit can beprovided.

From the disclosure thus described, it will be obvious that theembodiments of the disclosure may be varied in many ways. Suchvariations are not to be regarded as a departure from the spirit andscope of the disclosure, and all such modifications as would be obviousto one skilled in the art are intended for inclusion within the scope ofthe following claims.

What is claimed is:
 1. An electromagnetic wave shielding sheet,comprising: a protection layer, a metal layer, and a conductive adhesivelayer, wherein the metal layer has a plurality of openings, and anaperture ratio of the openings is in a range of 0.1% or more to lessthan 15%; and a tensile breaking strength is 10 N/20 mm-80 N/20 mm. 2.The electromagnetic wave shielding sheet according to claim 1, whereinan average value of a circularity factor of the openings of the metallayer obtained by an expression below is 0.5 or more,circularity factor=(area×4π)/(perimeter)² herein, the perimeter is alength of an outer circumference when an image which is obtained byobserving the metal layer by any one of an optical microscope, a lasermicroscope and an electron microscope is read, a plane of the openingbecomes a direction perpendicular to an observation viewpoint, theopening capable of being entirely confirmed is extracted, and aextracted opening is projected two-dimensionally; and the area is abreadth of a region defined by the outer circumference when theextracted opening is projected two-dimensionally.
 3. The electromagneticwave shielding sheet according to claim 1, wherein a thickness of themetal layer is 0.5 μm-5 μm.
 4. The electromagnetic wave shielding sheetaccording to claim 2, wherein a thickness of the metal layer is 0.5 μm-5μm.
 5. The electromagnetic wave shielding sheet according to claim 1,wherein the conductive adhesive layer contains a thermosetting resin anda conductive filler, and a content of the conductive filler in theconductive adhesive layer is 35 mass %-90 mass %.
 6. The electromagneticwave shielding sheet according to claim 2, wherein the conductiveadhesive layer contains a thermosetting resin and a conductive filler,and a content of the conductive filler in the conductive adhesive layeris 35 mass %-90 mass %.
 7. The electromagnetic wave shielding sheetaccording to claim 3, wherein the conductive adhesive layer contains athermosetting resin and a conductive filler, and a content of theconductive filler in the conductive adhesive layer is 35 mass %-90 mass%.
 8. The electromagnetic wave shielding sheet according to claim 4,wherein the conductive adhesive layer contains a thermosetting resin anda conductive filler, and a content of the conductive filler in theconductive adhesive layer is 35 mass %-90 mass %.
 9. A printed wiringboard comprising the electromagnetic wave shielding sheet according toclaim 1, a cover coat layer, and a wiring board having signal wiringsand an insulation base material.
 10. The printed wiring board accordingto claim 9, wherein the signal wirings have signal circuits and groundcircuits; a via is arranged on the cover coat layer to expose the groundcircuits; and an area of the via is 0.008 mm² or more and 0.8 mm² orless.